When boreholes are drilled to recover oil or gas the well casing dropped into the hole is usually cemented at the lower end of the hole, and at other locations above the lower end. In cementing the lower end of the casing, usually called primary cementing, a cement slurry is passed down through the casing and up into the annular space between the casing and the borehole. Cementing above the lower end of the borehole is usually done later than the primary cementing job, that is, during the productive life of the well. These later operations are usually referred to as secondary cementing, or stage cementing.
In a stage cementing operation, as the name suggests, the borehole annulus is cemented in separate stages, beginning above the primary cementing job and working up the borehole. Special tools have been developed for use in stage cementing to make the operation more convenient and to save on the amount of cement required. Most of these tools have an upper sleeve and a lower sleeve, which are slidable inside a collar having cement ports therein. In the "running in" position both sleeves are fastened to the collar by shear pins. In this position the cementing ports are closed off by the sleeves. To commence the cementing operation, a plug is flowed down the casing to seat on the lower sleeve. Fluid pressure is then applied behind the plug to shear the pins holding the lower sleeve and thus move the lower sleeve down to a point where the cementing ports are uncovered. Following completion of cementing, a second plug is seated on the upper sleeve and fluid pressure applied behind the plug shears the pins holding the upper sleeve. This allows the upper sleeve to move down and close off the cementing ports.
Stage cementing tools of the type described above frequently have operating problems. For example, when the second plug (the closing plug) seats on the upper sleeve several inches of cement slurry are trapped between the second plug and the first plug (the opening plug), which remains seated in the lower sleeve. When the upper sleeve moves downwardly, therefore, it must work against the trapped cement. Because of this obstacle, the amount of fluid pressure required to move the upper sleeve to its closed position can sometimes be very near to the burst pressure rating for the well casing.